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Full text of "Text Book Of Mechanical Engineering"

Appendix II.

869

The extreme cases were from -96 to 1-04 times these numbers,
but at certain loads the values became constant As p increased
IUL decreased, except with dry pine on pine, when the reverse
occurred. The effect of long contact was to slightly increase p
with the dry experiments, and markedly so with wet pine on
pine.

For low-speed friction, Morin's results may be accepted.
The method of experiment is to find a load P which will just
keep a weight W moving at a slow uniform speed on a level

SLOW  SPEEDS

on,

surface, Fig.  831:   then the ratio P:W = ^, and very regular
results are obtainable.

At high speed somewhat conflicting figures are found.
Apparently there is a decrease in friction with increase of speed
if the surfaces are dry (p. 556), or rather, at high speeds the
friction seems to approach the lubricated cases, most probably
due to a cushion of air drawn in between the surfaces. High-
speed lubricated experiments are troublesome, because the